Start by gathering all your ingredients. Make sure the cherries are pitted and that your raspberries are fresh or properly thawed.

In a medium saucepan, combine 1/4 cup of granulated sugar, cornstarch, and kosher salt. Add in the pitted cherries and raspberries. Bring the mixture to a boil over medium heat and let it bubble for one minute.

Stir in the fresh lemon juice and elderflower liqueur, mixing well before removing from heat.

Transfer the cooked fruit mixture into a blender or food processor and blend until smooth. Strain the purée through a fine-mesh sieve, discarding solids.

Measure out 1 1/4 cups of the purée and set aside.

In a heatproof bowl, combine the large whole egg, egg yolks, 1/2 cup of granulated sugar, and a pinch of kosher salt over a double boiler.

Whisk continuously until the mixture is warm, reaching about 160°F.

Remove the egg mixture from heat and whisk until it thickens and cools to room temperature.

Gently fold in one cup of the reserved berry purée into the egg mixture.

In a separate bowl, whip 3/4 cup of heavy cream until stiff peaks form and fold it gently into the berry-egg mixture.

Carefully spoon the semifreddo mixture into eight glasses or serving cups.

Cover the glasses with plastic wrap and freeze for at least 8 hours, or up to 3 days.